Nozzles are used in a wide variety of applications to inject one fluid into another fluid and promote efficient mixing of the two fluids. Such applications include, for example, chemical reactor systems, industrial burners in process furnaces, fuel injectors in gas turbine combustors, jet engine exhaust nozzles, fuel injectors in internal combustion engines, and chemical or gas injection in wastewater treatment systems. The objective in these applications is to promote vortical mixing and rapid dispersion of the injected fluid into the surrounding fluid. It is usually desirable to achieve this efficient mixing with a minimum pressure drop of the injected fluid.
The proper design of injection nozzles for burners in industrial furnaces and boilers is important for maximizing combustion efficiency and minimizing the emissions of carbon monoxide and oxides of nitrogen (NOx). In particular, tightening regulations on NOx emissions will require improved and highly efficient nozzle and burner designs for all types of fuels used in industrial furnaces and boilers. Burners in these combustion applications utilize fuels such as natural gas, propane, hydrogen, refinery offgas, and other fuel gas combinations of varying calorific values. Air, preheated air, gas turbine exhaust, and/or oxygen-enriched air can be used as oxidants in the burners.
Conventional turbulent jets can be used in a circular nozzle tip to entrain secondary or surrounding combustion gases in a furnace by a typical jet entrainment process. The entrainment efficiency can be affected by many variables including the primary fuel and oxidant injection velocity or supply pressure, secondary or surrounding fluid flow velocity, gas buoyancy, primary and secondary fluid density ratio, and the fuel nozzle design geometry. Efficient low NOx burner designs require nozzle tip geometries that yield maximum entrainment efficiency at a given firing rate or at given fuel and oxidant supply pressures. Higher entrainment of furnace gases followed by rapid mixing between fuel, oxidant gas, and furnace gases produce lower average flame temperatures, which reduce thermal NOx formation rates. Enhanced mixing in the furnace space also can reduce CO levels in the flue gas. If the nozzle design geometry is not optimized, the nozzle may require much higher fuel and/or oxidant supply pressures or higher average gas velocities to achieve proper mixing in the furnace and yield the required NOx emission levels.
In many processes in the chemical industry, the fuel supply pressure is limited due to upstream or downstream processes. For example, in the production of hydrogen or synthesis gas from natural gas by steam methane reforming (SMR), a reformer reactor furnace fired by a primary natural gas fuel produces a raw synthesis gas stream. After optional water gas shift to maximize conversion to hydrogen, a pressure swing adsorption (PSA) system is used to recover the desired product from the reformer outlet gas. Combustible waste gas from the PSA system, which typically is recovered at a low pressure, is recycled to the reformer as additional or secondary fuel. High product recovery and separation efficiency in a PSA system requires that blowdown and purge steps occur at pressures approaching atmospheric, and typically these pressures are as low as practical to maximize product recovery. Therefore, most PSA systems typically produce a waste gas stream at 5 to 8 psig for recycle to the reformer furnace. After a surge tank to even out cyclic pressure fluctuations and necessary flow control equipment for firing control, the waste gas supply pressure available for secondary fuel to the reformer furnace burners may be less than 3 psig.
For cost-effective control of NOx emissions from SMR process furnaces, the burners should be capable of firing at these low secondary fuel supply pressures. If the burners cannot operate at these low pressures, the secondary fuel must be compressed, typically using electrically-driven compressors. For large hydrogen plants, the cost of this compression can be a significant portion of the overall operating cost, and it is therefore desirable to operate the reformer furnace burners directly on low-pressure PSA waste gas as the secondary fuel.
Some commercially-available low NOx burners use active mixing control methods such as motor-driven vibrating nozzle flaps or solenoid-driven oscillating valves to produce fuel-rich and/or fuel-lean oscillating combustion zones in the flame region. In these burners, external energy is used to increase turbulent intensity of the fuel and oxidant jets to improve mixing rates. However, these methods cannot be used in all low NOx burner designs or heating applications because of furnace space and flame envelope considerations. Other common NOx control methods include dilution of fuel gas with recirculated flue gas or the injection of steam. By injecting non-reactive or inert chemical species in the fuel-oxidant mixture, the average flame temperature is reduced and thus NOx emissions are reduced. However, these methods require additional piping and costs associated with transport of flue gas, steam, or other inert gases. In addition, there is an energy penalty due to the required heating of dilution gases from ambient temperature to the process temperature.
It is desirable that new low NOx burner designs utilize cost-effective passive mixing techniques to improve process economics. Such passive techniques utilize internal fluid energy to enhance mixing and require no devices that use external energy. In addition, new low NOx burners should be designed to operate at very low fuel gas pressures. Embodiments of the present invention, which are described below and defined by the claims which follow, present improved nozzle and burner designs which reduce NOx emissions to very low levels while allowing the use of very low pressure fuel gas.